Struts2的错误FreeMarker template error!

FreeMarker template error!

Error on line 52, column 9 in template/simple/doubleselect.ftl
stack.findString(parameters.listValue) is undefined.
It cannot be assigned to itemValue
The problematic instruction:
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==> assignment: itemValue=stack.findString(parameters.listValue) [on line 52, column 9 in template/simple/doubleselect.ftl]
 in user-directive s.iterator [on line 45, column 1 in template/simple/doubleselect.ftl]
----------

Java backtrace for programmers:
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freemarker.core.InvalidReferenceException: Error on line 52, column 9 in template/simple/doubleselect.ftl
stack.findString(parameters.listValue) is undefined.
It cannot be assigned to itemValue
	at freemarker.core.Assignment.accept(Assignment.java:111)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.IfBlock.accept(IfBlock.java:82)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.MixedContent.accept(MixedContent.java:92)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.Environment.visit(Environment.java:233)
	at freemarker.core.UnifiedCall.accept(UnifiedCall.java:116)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.MixedContent.accept(MixedContent.java:92)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.Environment.process(Environment.java:176)
	at freemarker.template.Template.process(Template.java:232)
	at org.apache.struts2.components.template.FreemarkerTemplateEngine.renderTemplate(FreemarkerTemplateEngine.java:148)
	at org.apache.struts2.components.UIBean.mergeTemplate(UIBean.java:530)
	at org.apache.struts2.components.UIBean.end(UIBean.java:484)
	at org.apache.struts2.views.jsp.ComponentTagSupport.doEndTag(ComponentTagSupport.java:43)
	at org.apache.jsp.zsgj.add_jsp._jspx_meth_s_doubleselect_0(add_jsp.java:222)
	at org.apache.jsp.zsgj.add_jsp._jspx_meth_s_form_0(add_jsp.java:150)

错误原因：数据集中有空值。将改非空即可。